TOPOLOGIES
What is a Topology?
The arrangement of computers in a network is called Topology. In a network, computers can be connected in different ways:
Point to Point Topology
Bus Topology
Ring Topology
Star Topology
Point To Point Topology
This is the simplest possible network in which two end-systems are connected by a network link.
Bus Topology
Structure
It is described as a multi-point connection. In this topology, all the devices are connected to a common half-duplex link called a bus. Whenever a device wants to send a message to any other device, the message is broadcasted to all the devices connected to the main cable (which is the main cause of the larger number of collisions of messages in this topology).
How different components are connected in Bus Topology.
In the figure above, T is the terminator which absorbs the signals which arrive at the end to stop them from bouncing back in the main cable.
Advantages
Easier to set-up/extend
Less cable required
Disadvantages
Server cannot communicate with computers and routers at the same time.
Every computer can read packets intended for other computers.
If the main cable breaks, network performance degrades badly.
Difficult to detect and troubleshoot faults at an individual station.
Efficiency reduces as the number of devices connected to it increases.
Due to Collisions this topology is not suitable for networks with heavy traffic.
Security is lower because the messages are broadcasted.
Major Problem with Bus Topology
A collision happens when two devices sharing a network medium, like a bus, transmit at the same time and interfere with each other resulting in data loss.
Solution of collision problem in Bus topology
CSMA/CD
CSMA/CA
CSMA/CD(Carrier Sense Multiple Access (with) Collision Detection)
Before transmitting, a device checks if the channel is busy or free. If it is busy the device waits and if the channel is free, data is sent. When transmission begins the device listens for other devices also starting transmission. If there is a collision, transmission is aborted, transmitting a jam signal and both devices wait a (different) random time, then try again.
CSMA/CA(Carrier Sense Multiple Access (with) Collision Avoidance)
In computer networking, it is a network multiple access method in which carrier sensing is used, but nodes attempt to avoid collisions by beginning transmission only after the channel is sensed to be “idle".if the medium was identified as being clear or the node received a CTS (Clear To Send ) to explicitly indicate it can send, it sends the frame in its entirety.
Difference between CSMA/CD and CSMA/CA
Structure
A ring topology is a network configuration in which device connections create a circular data path. Each networked device is connected to two others, like points on a circle.In a ring network, packets of data travel from one device to the next until they reach their destination. Most ring topologies allow packets to travel only in one direction, called a unidirectional ring network. Others permit data to move in either direction, called bidirectional.
A token ring is a topology in which all devices are connected in a ring and pass one or more tokens from host to host. A token is a frame of data transmitted between network points. Only a host that holds a token can send data, and tokens are released when receipt of the data is confirmed. This topology was developed by IBM in the 1980s as an alternative to Ethernet to avoid the major problem of collision.
Advantages
Collisions cannot occur because only one computer holding the token can transmit.
Works well under heavy loading
Possible to form very large networks
No server is needed to control network connectivity between each workstation.
Disadvantagesfaulty connections can cause the whole network to fail
It is difficult to expand this type of network.
less secure (because data passes through all computers)
The entire network will be impacted if one workstation shuts down.
The hardware needed to connect each workstation to the network is more expensive than Ethernet cards and hubs/switches.
Star Topology
Structure
In a star topology, each end system has a point-to- point connection to the Central device. These end systems can be user workstations or servers. The central device can be used to connect the network to other networks and in particular to the internet.
Advantages
Server can send packets to different computers at the same time.
Network software on each computer don’t need to include collision detection and
avoidance.
computers cannot read data packets intended for any other computer.
Signals only go to the destination, so this topology provides a secure connection.
more than one communication can be done at the same time.
Easy to connect/remove nodes or devices; so easy to troubleshoot.
Centralised management helps in monitoring the network.
Failure of one node or link doesn’t affect the rest of the network.
Connections may use different protocols
Fewer collisions
Each device could use a different type of line / cable
Disadvantages
If the central device fails then the whole network goes down.
Performance is dependent on the capacity of the central device.
How are computers connected to switches and servers in Star topology?
Connecting a Start Network to the Internet
If a star network needs to be connected to the internet, it can be connected through a router to either a server or a Switch.
Advantages of using Server to connect start network to internet
It can provide a (software) firewall
The server can check all internet traffic
Server acts as proxy
Advantage of using Switch to connect start network to internet
Internet traffic by- passes the server
Server not overloaded with internet traffic
connected to all computers
